In recent years, in TFT substrates included in liquid crystal display devices, as switching elements of pixels each of which is a minimum unit for forming an image, TFTs using semiconductor layers made of an oxide semiconductor and having satisfactory characteristics such as high mobility, high reliability, and low off currents have been proposed instead of conventional TFTs using semiconductor layers made of amorphous silicon.
A general TFT having a bottom gate structure includes, for example, a gate electrode provided on a glass substrate, a gate insulating film provided to cover the gate electrode, a semiconductor layer provided on the gate insulating film and above the gate electrode, a source electrode and a drain electrode provided on the semiconductor layer to be spaced apart from each other. In part of the semiconductor layer which is exposed between the source electrode and the drain electrode, a channel region is formed. The TFT having the bottom gate structure is covered with a protective insulating film, and a pixel electrode formed on the protective insulating film is connected to the drain electrode via a contact hole formed in the protective insulating film.
A TFT substrate including the TFT having the bottom gate structure as described above can be fabricated by, for example, repeating a series of steps of sequentially forming an etched film and a photosensitive resin film on the glass substrate, exposing the photosensitive resin film via a photomask and then developing the photosensitive resin film to form a resist pattern, and etching the etched film exposed form the resist pattern. The same applies to the TFT substrates including the TFTs using the oxide semiconductor layers.
For example, Patent Document 1 describes a method for fabricating a TFT substrate including a TFT having a bottom gate structure using an oxide semiconductor layer by using five photomasks. In this method, by using a first photomask, a gate electrode is formed on a glass substrate, and a gate insulating film is formed o cover the gate electrode. Then, by using a second photomask, the oxide semiconductor layer is formed. Subsequently, by using a third photomask, a source electrode and a drain electrode are formed. Next, a protective insulating film (interlayer insulating film) is formed to cover the source electrode and the drain electrode, and by using a fourth photomask, a contact hole is formed in the protective insulating film. Finally, by using a fifth photomask, a pixel electrode is formed. The TFT substrate can thus be formed.